Solenoid driven clutches of the type commonly used with appliances and automotive accessories must establish smooth torque transfer between motor and load to minimize noise and wear. When a clutch coil is energized by a fixed magnitude voltage source, the rate of rise of coil current is a function of the R-L time constant of the coil and driving circuitry; saturation current is limited by coil resistance.
Following initial energization of the coil, electromagnetic flux developed by the coil causes the armature of the clutch to move in a direction to reduce the solenoid air gap within a magnetic circuit constituting the coil, armature and gap. During initial movement of the armature, there is a dip or "decrease" in coil current; thereafter, current continues to rise at a slightly lower rate determined by the increased value of inductance of the coil caused by the smaller air gap. Because the force applied to the armature by the magnetic field produced by the coil is non-linear and substantially uncontrolled, torque transfer between the plates of the clutch tends to be abrupt and noisy, and there is excessive wear of the plate surfaces.